The invention relates to a method and a machine for cleaning objects having the general form of plates, such as screens or stencils for screen printing, notably screen printing stencils used in the production of electronic circuits for depositing adhesives or solder pastes, and more particularly stencils for frames, so-called self-tensioning frames, which are simple flexible fragile thin metal foils.
So far, cleaning of such pieces is often realized in practice manually with the help of solvents on a duster for the glue, or with the help of machines (DE-A-4,010,679) spraying under pressure on the pieces either aqueous solutions with saponification agents or solvents (with all the technical strains imposed by the use of chemicals with an ignition point). Such devices are cumbersome, represent important investments, and do not allow to clean and separate in one step residues of various chemical compositions. However, modern regulations on environment impose to be able to separate residues to allow their recycling, their inerting, or their later destruction according to their chemical composition.
So, with these devices, it is necessary to provide a rinsing step and a residue sorting/separating step. Moreover, the cleansing solutions load with residues and must be regularly renewed, which is expensive and harmful to environment.
Further, these devices do not allow to clean the stencils for self-tensioning frames, which are too fragile for a manual rub cleaning or a pressure spray cleaning.
So, effective technical cleansing compositions are known such as VIGON(copyright) SC 200 marketed by the company Dr. O.K. WACK for spray cleaning of screen printing stencils, which has the advantage of eliminating at room temperature adhesives used for surface mounting of components (CMS) and solder pastes, and which is not inflammable. This composition based on water-dissolved alcoxypropanols forms a microphase at a temperature between 20 and 30xc2x0 C. It is recommended by the manufacturer, after applying the fluid by spraying on objects to be cleaned, to rinse with water. With these compositions, it is noted that the spray-treatment time for each piece is in general more than 1 hour, drying included.
Document PATENT ABSTRACTS OF JAPAN, vol. 95, no. 8, Sep. 29th 1995 and JP 07116617 describes a cleaning method and a cleaning machine in which an object is immersed in a cleansing liquid which flows from the bottom up in the treating vessel, spills over into a peripheral overflow vessel, carrying along residues in suspense, and is recycled, after filtering, in the treating vessel bottom part. Precipitated residues of high granularity are extracted because of a high flow rate from the bottom part of the treatment vessel, and then filtered and recycled. This device does not allow in practice to clean pieces and to separate residues of low density, of residues of average density and of residues of high density with an efficiency and a time compatible with industrial operating stresses. Particularly, this device does not allow to separate residues from successive or alternative treatments of pieces soiled with solder pastes (light and heavy residues), and then of pieces soiled with adhesive (residue of average density). Particularly, it is to be noted that the device according to this document does not particularly allow to separate residues of low density from other residues, notably from residues of average density in suspense. This separation is indeed the most difficult one to realize, but it is important, notably insomuch as residues of low density such as fluxes or fats have a chemical composition very different from that of residues of average density such as epoxy glue, and thus need later different treatments. Further, this device which does not comprise any heating means does not allow to use a liquid susceptible of forming a microphase. It works thus either as a solvent, or as a surfactant. Consequently, in such a device, the cleansing liquid dissolves residues, and must thus be regularly renewed, which is expensive and harmful to environment.
The invention aims thus to solve these drawbacks by providing a method and a machine for cleaning objects having the general form of plates, such as screen printing stencils, which can be soiled with residues of average density, notably of density of the order of 1, and/or with solid residues of high density, notably of density not less than 1, and/or with residues of low density, notably of density less than 1, and this under economical conditions compatible with an industrial use, and allowing to separate at least residues of low density.
More particularly, the invention aims to allow to automatically separate residues according to their density, after indifferently successive or alternative treatments of a number of different pieces soiled with residues of any density. Particularly, the invention aims to allow to alternatively treat pieces soiled with residues of low density and with residues of high density, such as screen printing stencils soiled with solder pastes (including fluxes of low density and metal alloy particles of high density), and pieces soiled with residues of average density such as screen printing stencils soiled with glue (residue of average density), and to separate these different residues according to their density at the time of each treatment, without maintenance between two treatments of distinct pieces.
The invention aims thus to allow to treat successively or alternatively pieces soiled with various residues (indifferently of low, high or average density), without any waiting time between pieces.
Particularly, the invention aims to allow to avoid to realize the distinct subsequent steps consisting in rinsing and separating residues, and permanently renewing the used cleansing composition.
The invention aims also to provide a method and a machine which has a low cost-investment, is simple and economical to use and provides a high cleansing efficiency. Particularly, the invention aims to enable a complete treatment (cleansing and drying) with a treatment time less than 1 hour, notably of the order of 0.5 hour, per object.
The invention aims to enable to clean any kind of objects, included fragile objects, such as stencils for self-tensioning frames, soiled with various very resistant residues adhering to the object such as metal particles (solder alloys), solder fluxes, fats, adhesives (epoxy glues, adhesives for CMS, . . . ).
For so doing, the invention relates to a method for cleaning objects having the general form of plates, such as screen printing stencils susceptible of being soiled with residues of average density such as adhesive particles, and/or with solid residues of high density such as metal particles, and/or with residues of low density such as solder fluxes or fats, in which at least one object to be cleaned is immersed in a vessel, so-called treating vessel, filled with a liquid cleansing composition, at least one of vertical walls of the treating vessel, so-called overflow wall, being adapted to form an overflow container into a second adjacent vessel, so-called overflow vessel, the overflow wall being common to the treating vessel and the overflow vessel, at least one flow Q1 of cleansing composition is continuously extracted from the treating vessel and reintroduced into the treating vessel after a passage through filtering means for filtering residues, and a flow Q2 of cleansing composition is continuously extracted from the overflow vessel and reintroduced into the treating vessel after a passage through filtering means for filtering residues, characterized in that:
it is used a cleansing composition formed of a liquid azeotropic solution of a density at least substantially equal to the density of residues of average density, adapted to loose residues from the object without solubilizing them,
at least one object is placed vertically into a treating vessel adapted to have at least substantially vertical walls, so-called vertical longitudinal walls, spaced by a distance less than 10cm, from the main faces, disposed at least substantially vertically, so-called vertical faces, of an object to be cleaned.
the cleansing composition is continuously extracted from the bottom part of the treating vessel and is continuously extracted from the overflow vessel, and the whole cleansing composition extracted from both vessels is continuously reintroduced in the top part of the treating vessel, after a passage through said filtering means, the object being immersed into the cleansing composition which continuously flows in the treating vessel along the object downwards with a flow Q1 adapted to:
allow the cleansing composition to produce its effects on residues by loosing them from the object,
allow the cleansing composition to carry along residues of average density and residues of high density,
allow flotation of residues of low density and their overflow separation into the overflow vessel, so that in one continuous step residues are loosed from the object, at least residues of low density are separated, and residues of the cleansing composition which does not load with residues are extracted.
It is to be noted that a method according to the invention is advantageously applicable to objects having at least as a whole the form of plane plates, the treating vessel having the form of a parallelepiped. Nevertheless, the invention is also applicable to objects which have not strictly the form of plane plates (wavy, cylindrical or hemi-cylindrical plates . . . ). The form of the walls of the treating vessel is adapted to that of objects to be cleaned, so as to have vertical longitudinal walls with forms substantially corresponding to those of main vertical faces of the objects. Advantageously, the objects have main faces in the form of ruled surface (plane, cylinder or portion of cylinder with a circular base or with a base of any form), and the vertical walls of the treating vessel have a similar form.
As the cleansing composition is azeotropic, it is eliminated from the object by simple evaporation (by natural convection or by hot-air drying) without trace. As its density corresponds to that of residues of average density, it carries along these residues in its movement. Residues of high density flow to the bottom of the treating vessel and are carried along with the cleansing composition extracted from the bottom part of the treating vessel.
The cleansing composition is adapted not to solubilize residues, and thus does not load with residues. Advantageously and according to the invention, it is used a cleansing composition formed of an aqueous solution having the effect of loosing residues from the object without solubilizing them.
Advantageously and according to the invention, the cleansing composition is an uninflammable solution.
In the case of screen printing stencils, residues of average density are glues of density close to 1, residues of low density are fluxes of solder pastes or fats, residues of high density are particles of metal weld or solder alloys or those of conductive glues. Advantageously and according to the invention, it is used a cleansing composition the density of which is of the order of 1.
Advantageously and according to the invention, it is used a cleansing composition formed of an aqueous solution of at least one constituent forming a microphase, notably at least an alcoxyalcohol, at a temperature less than 50xc2x0 C., and the cleansing composition is maintained at a treatment temperature adapted to enable the microphase formation, notably less than 50xc2x0 C. It is known indeed that in such an aqueous solution in the microphase state, microbubbles of alcoxyalcohol(s) are formed in water, and have an important decomposition power with regard to residues by suppressing their adherence on the support, without nevertheless solubilizing them as a surfactant. For example, it can be used a composition formed of approximately 90% (in weight) and 10% (in weight) of at least one constituent among a propylen glycol ether, such as propylen glycol methyl ether and/or a dipropylen glycol monoether; a polypropylen glycol ether, an alcoxypropanol, such as ethoxypropoxypropanol and/or n-butoxypropanol. Particularly, advantageously and according to the invention, it is used an aqueous solution saturated with at least a cleaning agent as described in U.S. Pat. No. 5,486,314.
As the cleansing composition can be used at low temperature (less than 50xc2x0 C., notably between 20xc2x0 C. and 30xc2x0 C.), any polymerization, reticulation or hardening process, started by residues heating, is avoided, which helps the cleaning.
Advantageously and according to the invention, VIGON(copyright)SC200 marketed by the firm Dr O.K. Wack Chemie GmbH (Ingolstadt, Germany) is used as a cleansing composition, the exact chemical composition of which is not known. The density of this cleansing composition is equal to 1. Nevertheless, it is to be noted that, contrary to manufacturer""s recommendations, the inventor has surprisingly noted that this composition is highly efficient when used in bath form, i.e. even in the absence of spray supply of mechanical power. It forms a microphase between 20xc2x0 C. and 30xc2x0 C.
In other respects, advantageously and according to the invention, the cleansing composition is supplied again by spraying on the free surface of the cleansing composition contained in the treating vessel, from at least one spraying ramp, disposed at a height superior to that of the overflow wall. Advantageously and according to the invention, the cleansing composition is diffused across said free surface. Advantageously and according to the invention, it is used two spraying ramps parallel one to another, jets formed by one ramp intersecting those formed by the other ramp immediately over or at the level of said free surface so as to form a cleansing composition screen having the effect of rinsing an object being extracted from the treating vessel.
Advantageously and according to the invention, the flow Q1 of cleansing composition extracted from the treating vessel is adapted so that the retention time in the treating vessel is comprised between 30 seconds and 5 minutes, notably of the order of 2 to 4 minutes. This flow Q1 is adapted to allow the cleansing composition to carrying along residues of average and/or high density, but also to enable the counter-flow flotation of residues of low density and their filter separation through specific filtering means, and for the cleansing composition to be able to produce its effects on the residues. The value of this flow Q1 can be empirically adjusted by means of tests according to the residue nature, the form of the object and of the treating vessel.
Advantageously and according to the invention, it is used a treating vessel adapted to have vertical longitudinal walls spaced from the vertical main faces of the object by a distance comprised between 1 and 5 cm, notably of the order of 2 to 3 cm. The distance separating the vertical walls of the treating vessel from the faces of the object is as reduced as possible, so as to realize as far as possible in the treating vessel a slow and homogenous flow free of cleansing composition turbulence, and to minimize the quantity of cleansing composition used in the machine.
Indeed, contrary to what the state of the art indicates, the use of a small quantity of cleansing composition has many advantages. As such, it is to be noted that the time necessary for the cleansing composition to be completely filtered and free of all its residues after a treatment operation of a piece is very short (of the order of one minute). Every new piece to be treated is thus immersed into a cleansing composition free of any contamination. Pieces soiled with residues varying from one piece to another can thus successively or alternately be treated without taking risk in contaminating one piece with the residues of another piece that has already been treated, or of interaction of residues of one piece with those of another piece that has already been treated.
Further, advantageously and according to the invention, ultrasonic waves are emitted in the treating vessel in one direction at least substantially perpendicular to the main plane of an object placed in the treating vessel.
Particularly, these ultrasonic waves facilitate the action of the cleansing composition in the interstices or the microperforations in the object.
The invention also applies to a machine for working a process according to the invention.
The invention also relates to a machine for cleaning objects having the general form of plates, such as screen printing stencils, susceptible of being soiled with residues of average density such as glue particles, and/or with solid residues of high density such as metal particles, and/or with residues of low density such as solder fluxes or fats, comprising a vessel, so-called treating vessel, adapted to be able to contain a liquid cleansing composition, and at least an object to be cleaned immersed into this cleansing composition, at least one of the vertical walls of the treating vessel, so-called overflow wall, being adapted to form an overflow container into a second adjacent vessel, so-called overflow vessel, the overflow wall being common to the treating vessel and the overflow vessel, means, so-called first circulation means, for continuously extracting at least one flow Q1 of cleansing composition from the treating vessel and for continuously reintroducing it into the treating vessel after a passage through filtering means for filtering residues, and means, so-called second circulation means, for continuously extracting a flow Q2 of cleansing composition from the overflow vessel, and for continuously reintroducing it into the treating vessel after a passage through filtering means for filtering residues, characterized in that:
the treating vessel is adapted to receive at least one object to be cleaned in a vertical position, and to have at least substantially vertical walls, so-called vertical longitudinal walls, spaced by a distance less than 10cm from main faces, so-called vertical faces, disposed at least substantially vertically, of an object to be cleaned,
first circulation means are adapted to continuously extract the cleansing composition from the bottom part of the treating vessel and to continuously reintroduce it in the top part of the treating vessel,
second circulation means are adapted to reintroduce in the top part of the treating vessel the cleansing composition extracted from the overflow vessel,
first circulation means are adapted to maintain in the treating vessel and along the object, a flow Q1 of cleansing composition flowing downwards and adapted to:
allow the cleansing composition to produce its effects on the residues by loosing them from the object,
allow the cleansing composition to carry along residues of average density and residues of high density,
allow flotation of residues of low density and their overflow separation into the overflow vessel, so that as the cleansing composition is formed of a liquid azeotropic solution of a density at least substantially equal to the density of residues of average density, and is adapted to loose the residues from the object without solubilizing them, in one continuous step, residues are loosed from the object, at least residues of low density are separated, and residues of the cleansing composition which does not load with residues is extracted.
Advantageously, the machine according to the invention comprises at least one spraying ramp for spraying the cleansing composition onto the free surface of the cleansing composition contained in the treating vessel, this spraying ramp being disposed at a height superior to that of the overflow wall. Advantageously and according to the invention, it comprises two spraying ramps parallel one to another, jets formed by one ramp intersecting those of the other ramp immediately above or at the level of said free surface so as to form a cleansing composition screen having the effect of rinsing an object being extracted from the treating vessel. Advantageously and according to the invention, the spraying ramps are disposed at least substantially above and perpendicularly to the extreme upper edges of two vertical longitudinal walls of the treating vessel, at least one of which forms the overflow wall. Advantageously and according to the invention, a first spraying ramp belongs to first circulation means and a second spraying ramp belongs to second circulation means.
Advantageously and according to the invention, the overflow wall is formed of one of the vertical longitudinal walls of the treating vessel the extreme upper edge of which is at a height inferior to that of the extreme upper edges of the other vertical walls of the treating vessel.
Advantageously and according to the invention, the treating vessel is adapted to have vertical longitudinal walls spaced from vertical main faces by a distance comprised between 1 and 5 cm, notably of the order of 2 to 4 cm.
Advantageously and according to the invention, the machine comprises emission means for emitting ultrasonic waves associated to at least one of the vertical longitudinal walls of the treating vessel so as to emit ultrasonic waves in the treating vessel in a direction at least substantially perpendicular to a main plane of an object placed in the treating vessel.
Advantageously and according to the invention, first and/or second circulation means comprise thermoregulated heating means for heating the cleansing composition, disposed below filtering means for filtering residues, and above reintroduction means for reintroducing the cleansing composition in the top part of the treating vessel.
Advantageously and according to the invention, first circulation means comprise a filter, so-called first filter, and a pump, so-called first pump, and second circulation means comprise a filter, so-called second filter, and a pump, so-called second pump. The first and second filters are similar, and the first and second pumps are similar.
It is to be noted that first circulation means can comprise only one outflow port for the cleansing composition in the bottom part of the treating vessel, and only one filter able to filter residues of average density and to retain residues of high density, while separating them, notably by decantation. For example, this filter can be formed of a container including a cylindrical filtering cartridge through which a liquid to be filtered radially passes. This variant is applicable advantageously when pieces, such as stencils soiled with solder pastes and then pieces soiled with glue are equally well successively or alternately treated. Indeed, in this case, the same filter retains in the container bottom part residues of high density (particles) which it receives when the piece being treated is soiled with solder paste (these residues falling down to the bottom of the container under gravity), and then retains in its cartridge residues of average density which it receives when the piece being treated is soiled with glue (residues being in suspense in the liquid). As such, it is to be noted that, as indicated above, the small quantity of cleansing composition allows to treat successively or alternatively pieces soiled with various residues, with a short waiting time between each piece.
In a variant, two separate outflow ports can be provided, one of which in the bottom part of the treating vessel supplies a filter able to filter residues of high density, whereas the other, situated at an upper level (above the bottom wall of the treating vessel, but nevertheless in the lower part of the treating vessel), supplies another filter able to filter residues of average density. In another variant, an outflow port can be advantageously provided at half height in one of the vertical longitudinal walls of the treating vessel so as to generate a liquid composition flow at least substantially perpendicular to the vertical faces of the object, e.g. in order to facilitate the passage of the cleansing composition through the through ports or perforations in the object and the carrying-along of residues which are incrusted therein.
In all cases, residues of low density are separated in a distinct specific filter of second circulation means.
Advantageously and according to the invention, the treating vessel has the form of a parallelepiped, the width of which is comprised between 3 cm and 10 cm, notably of the order of 5 cm or 6 cm, and has a volume comprised between 10 l and 100 l, notably of the order of 50 l. The height of the treating vessel is at least equal to that of the objects to be cleaned.
Further, advantageously and according to the invention, the machine comprises a suspension frame for suspending an object having the form of a flexible plate in the treating vessel, this suspension frame being adapted to receive and maintain the peripheral edges of the object.
The invention also relates a method and a machine characterized in combination by all or part of the above-mentioned or below-mentioned features.